Differential Equations and Linear Algebra (4th Edition)

Published by Pearson
ISBN 10: 0-32196-467-5
ISBN 13: 978-0-32196-467-0

Chapter 8 - Linear Differential Equations of Order n - 8.8 A Differential Equation with Nonconstant Coefficients - Problems - Page 567: 14

Answer

$y(x)=c_1x^{-1}+c_2x^{-2}+\frac{1}{2}\ln x-\frac{3}{4}$

Work Step by Step

Given $x^2y''+4xy'+2y=4\ln x$ In this case the substitution $y(x) = x^r$ yields the indicial equation $$r(r-1)+4r+2=0\\ r^2+3r+2=0\\ (r+1)(r+2)=0$$ It follows that two linearly independent solutions to the given differential equation are $u_1(x)=x^{-1}\\ u_2(x)=x^{-2}$ so that the general solution is $y(x)=u_1(x)x^{-1}+u_2(x)x^{-2}$ where $u_1$ and $u_2$ are determined from $u_1'(x)x^{-1}+u_2'(x)x^{-2}=0\\ u_1'(x)(x^{-1})'+u'_2(x)(x^{-2})'=4x^{-2}\ln x$ Hence, $u_1'=4\ln x\\ u_2'=-4x\ln x$ which upon integration gives $u_1(x)=4(x\ln x -x)\\ u_2(x)=-4(\frac{x^2}{2}\ln x+\frac{x^2}{4})$ Thus, the given equation has general solution $y(x)=c_1x^{-1}+c_2x^{-2}+\frac{1}{2}\ln x-\frac{3}{4}$
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